Introduces new API endpoint /api/slack to allow Slack Notifications through a POST Request to a channel in Slack.

This endpoint requires a type GET parameter with one of the following values:

Each key in the request body will result in one new line in the slack message allowing us to send a formatted text. Different types will notify different people in our Slack channel. We can choose to use different channels by configuring different endpoints in our code using the "Incoming Webhook" settings of Slack.

curl "https://{vercel-domain}/api/slack?type=message" \
-X POST \
-H "Content-Type: application/json" \
-d "{\"a\":\"This is a text\",\"b\":\"More text\"}"

// results in a Slack message like:
NEW MESSAGE: <@userid>
a: this is a text
b: More text